1. Field of Invention
The present invention generally relates to motors with piezoelectric transducer elements, and more particularly, to the electrical and mechanical mounting of a high deformation piezoelectric transducer within a motor housing.
2. Description of the Prior Art
Piezoelectric and electrostrictive materials develop a polarized electric field when placed under stress or strain. Conversely, they undergo dimensional changes in an applied electric field. The dimensional change (i.e., expansion or contraction) of a piezoelectric or electrostrictive material is a function of the applied electric field.
A typical prior ceramic device such as a direct mode actuator makes direct use of a change in the dimensions of the material, when activated, without amplification of the actual displacement. Direct mode actuators typically include a piezoelectric or electrostrictive ceramic plate sandwiched between a pair of electrodes formed on its major surfaces. The device is generally formed of a sufficiently large piezoelectric and/or electrostrictive coefficient to produce the desired strain in the ceramic plate. However, direct mode actuators suffer from the disadvantage of only being able to achieve a very small displacement (strain), which is, at best, only a few tenths of a percent.
Indirect mode actuators are known in the prior art to provide greater displacement than is achievable with direct mode actuators. Indirect mode actuators achieve strain amplification via external structures. An example of an indirect mode actuator is a flextensional transducer. Prior flextensional transducers are composite structures composed of a piezoelectric ceramic element and a metallic shell, stressed plastic, fiberglass, or similar structures. The actuator movement of conventional flextensional devices commonly occurs as a result of expansion in the piezoelectric material which mechanically couples to an amplified contraction of the device in the transverse direction. In operation, they can exhibit several orders of magnitude greater displacement than can be produced by direct mode actuators.
The magnitude of the strain of indirect mode actuators can be increased by constructing them either as "unimorph" or "bimorph" flextensional actuators. A typical unimorph is a concave structure composed of a single piezoelectric element externally bonded to a flexible metal foil, and which results in axial buckling or deflection when electrically energized. Common unimorphs can exhibit a strain of as high as 10% but can only sustain loads which are less than one pound. A conventional bimorph device includes an intermediate flexible metal foil sandwiched between two piezoelectric elements. Electrodes are bonded to each of the major surfaces of the ceramic elements and the metal foil is bonded to the inner two electrodes. Bimorphs exhibit more displacement than comparable unimorphs because under the applied voltage, one ceramic element will contract while the other expands. Bimorphs can exhibit strains up to 20% (i.e. about twice that of unimorphs), but, like unimorphs, typically can only sustain loads which are less than one pound.
A unimorph actuator called "THUNDER", which has improved displacement and load capabilities, has recently been developed and is disclosed in U.S. Pat. No. 5,632,841. THUNDER (which is an acronym for THin layer composite UNimorph ferroelectric Driver and sEnsoR), is a unimorph actuator in which one or more pre-stress layers are bonded to a thin piezoelectric ceramic wafer at high temperature, and during the cooling down of the composite structure asymmetrically stress biases the ceramic wafer due to the difference in thermal contraction rates of the pre-stress layers and the ceramic layer.
In operation a THUNDER actuator may be energized by an electric power supply via a pair of electrical wires which are typically soldered to the metal prestress layers or to the electroplated faces of the ceramic layer. In practice, these actuators 100 have been used to directly drive a pressure plate 8 or other mechanism in a prior art cyclic motor as shown in FIG. 3.
Typically, the convex face 100y of the actuator 100 would directly push (in the direction of arrow 7) against a plate 8 at the lowest point of its curvature, and the plate 8 would maintain contact with the actuator 100, returning to its rest position through the use of a spring mechanism 6. If multiple actuators are used, the actuators and their electrical connections were electrically isolated from each other using TEFLON.TM. tape (not shown).
A problem with the above described mounting method for a direct drive actuator is that the force against the actuator was concentrated on one point, or at least in a very small area of the actuator. This would cause the ceramic in the actuator or the whole actuator to break due to point load concentration. The actuator would then lose most of its effectiveness because it could not generate as much force or displacement with a cracked ceramic.
Another problem with prior art actuator mounting methods is that a single actuator typically could not generate sufficient force for higher output applications. This is especially true of applications where the pressure plate against which the actuator acted was spring mounted. The actuator dissipated a large amount of its useful force in trying to overcome the spring mechanism. The force generated for some applications would also fracture the ceramic layer of the actuator.
Another problem with prior art actuator mounting methods is that a single actuator typically could not generate sufficient displacement for higher output applications.
Another problem arose in prior applications using multiple, stacked actuators. In this application, the actuators are electrically isolated from each other using layers of TEFLON.TM. between the electrical connections on each actuator. The TEFLON.TM. also adds weight and opposes the motion of the actuator, dissipating the useful force and displacement in the stacked actuators.
Another problem with previous mounting methods for piezoelectric actuators is that wires which were soldered onto the electrode surface of the actuator would loosen and fracture as the actuator deformed. As the actuator deformed, the wire at its point of attachment would bend and eventually fracture from fatigue. The cyclic shaking motion of the actuator would also cause the soldered attachment point to loosen or fracture.